Just brought my new Raleigh Super Course back from the dead, but now it has the annoying habit of grinding the chain on the front derailer on the right-side downstroke.

I've checked BB tightness - no wobble (I tightened until stiff, then backed off just enough to eliminate the "grindy"feeling and smooth it out).

I've tightened the crank to a bit beyond what it should be (on a greased spindle, so no sticking).

I've adjusted the front derailer about three times now (kicked out, in, and dead straight...it's now perfectly in line with the chain in high gear). Based on the wooden school ruler one of my students left in the classroom, it's about 3.4mm away from the chain.

When you're spinning (little pressure), the chainrings run perfectly straight, or near enough you can't tell a wobble, so I know it's not that...

Am I missing something, or am I just a fatty that needs to learn how to spin more? I'm assuming it's not frame flex, as it's only 5 inches, and it's straight gauge 531 (it's a 1978 model)

Also, for maintenance sake, I'd like to replace it with a Shimano UN54 BB, but am not sure the spindle length for the crank - it's a standard Raleigh branded Sugino (I think). Any guesses on the length I'd need?

Check for loose chainring bolts. Thats all I can think of.

you never mentioned the crankset you have!
your weight, picture....

Yes I did - it's a Raleigh branded crank, that I'd guess is a Sugino (as it looks a lot like a Super Maxy that I have from an '81 Schwinn). Weight is 225. Picture is below.

Are the chainrings running true? It's relatively easy easy to knock the spider/large chainring of a swaged crankset out of alignment.

Like I said, when spinning it backward at a standstill, or even when spinning down the road, there isn't any noticeable movement.

I've noticed, with a variety of cranks, that when I'm putting a lot of force on the pedals, the chain and chainring will move a little in at one part of the stroke--if the derailleur is a little too much to the outside it rubs on the inside--and a little out at another part of the stroke--if the derailleur is a little too much to the outside. Trimming the derailleur fixes the problem, but I'm not sure exactly what's flexing. It might be the frame itself. A very small movement of the bottom bracket might perhaps be enough to move the chainring a millimeter or so. No doubt the chainrings aren't perfectly true side-to-side, but the fact that this only happens when I'm pushing hard on the pedals seems to suggest that something is flexing.

With torque, you might be flexing the actual chainring inward on the down stroke. There is quite a lot of force involved with the crank arm itself, and the pivot point being at the crank center, and splitting two of the spider arms leaves some possibility of force flexation. (This is pure speculation, by the way.) Double check your FD... the photo doesn't convince me that it is 100% aligned and under power you'll experience different results than spinning or turning the crank by hand on a stand.

Swaged cranksets, by design, are more susceptible to chainring deflection than a crankset with a one piece arm and spider.

correct- i have a supermaxy set with the big chainring swaged on. it has clear dish from flexing and im only 170 pounds.
sometimes i could hear a creak creak
inspect your chainline- maybe a shim or spacer adjustment could help

Is the chain rub on the inside edge or outside edge of the fd cage? What shifters? Is the front shifter tightened down enough? I had this problem recently with dt shifters. Evidently I was flexing the frame enough to pull a bit of cable, so the fd would travel inward and I'd get chain rub on the outside part of the cage.

Simple test: when the rub occurs, if you don't trim the fd and sit down and pedal easy, does it still rub? In my case it did, which indicated that the fd had actually changed position. If the rubbing only occurs when you're standing, and goes away if you sit back down, you have a different problem than the one I had.

Also, I don't understand this statement - what's the 5" you refer to?

That's a swaged crank; those are more prone to flex than forged cranks. Depending on how much you want to spend, you might try a new or used forged crank to see if that helps.

Sometimes the FD cage gets squeezed together. From the pictures, that doesn't seem to be the case. However, you might try to bend the inner part of the cage apart, just a little, and carefully. Spreading the plates apart will give you more room for flex. My guess is that this is a flex issue.

S.R. (Sakae Ringyo) is the crankset OEM. I had a 1978 Super Course and never had a flex problem. But I'm a lightweight.

The bike is completely stock - Raleigh branded Suntour power ratchet bar end shifters, Raleigh branded derailers (Suntour Cyclone rear, I'd guess the same front). It's got new cables, and the shifters don't creep out of alignment. The rubbing only occurs when pushing relatively hard - doesn't matter if sitting or standing, but it's less so when sitting (less pressure on the cranks). The 5" I'm referring to is the 5" between the derailer and the bottom bracket - there isn't really enough frame there to flex.

I've noticed this flex on an old and swaged Sugino crank. Sheldon Brown has a pretty good description of what a swaged crank is just scroll down to "swaging"

+1 on possible more flex with the swaged crankset, as there is less material on the spider where it meets the crank arm....
It could be frame flex too. I don't think you mentioned your frame size, but if your frame is quite large, you sill get more frame flex than usual. I get a bit of FD cage/chain rub on my Peugoet PSV when I climb hard on it, no matter how much I fiddle around and adjust my FD. It definitely comes from my frame being on the flexy side with the Supervitus tubing. I just learned to live with some chain/FD rub every time I did biggish climbs. You might have stiffer tubing than Supervitus on your bike, but when the frame is quite large you can get more flex from it......
Also make double sure that your FD cage is not bent. The outside face of the FD cage should be parallel to the chainrings. If they are bent, they are easy enough to bend back into alignment, just be gradual/gentle with doing it so you don't break the mechanism.

Chombi

Assuming it is frame flex, based on my experience it is generally the rear triangle that causes such issues. When the rear triangle flexes, the bb shell moves relative to the rear wheel. This causes the chain angle to change relative to the front derailleur and can cause rubbing. This situation is typically most evident when standing on climbs as the rocking motion tends to increase the deflection.

In all probability, your situation is probably a combination of factors. The problem is determining the major contributor. It could be the swaged crank or frame flex.

I've bent more arms of those Raleigh-branded SR cranksets than any other, and I dare say the chainring can't be far off in terms of durability.

Replace it with something more durable, and swap the high-normal Suntour Spirt front derailer in the process - the problem should disappear.

-Kurt